The blank meme is from Roothie: http://rooteh.deviantart.com/art/SONIC-FORMS-MEME-x-tended-161042268
Normal - The regular, official form (preferably modern design)
Knight - As a Knight of the Round Table (Sonic and the Black Knight)
Wereform - Werehog, Werecat or whatever applies (Sonic Unleashed)
Arabian Nights - As a genie or simply in Arabian clothing... You decide!
Super - The character's Super Mode (also accepted are Hyper form and super forms as Turbo Tails and Burning Blaze)
Robot - Either a robotic doppelganger (e.g. Metal Sonic) or a robotized version of the character.
Riders - In a Riders outfit (Sonic Riders (Zero Gravity))
Mephiles - Yessss! Imagine it was't Shadow whose appearance Mephiles copied (except you're doing Shadow, of course), but the character you picked Need reference? [link]
Classic - Classic design, that means round and chubby! (or e.g. the early Archie/SatAM designs)
Humanized - aka Gijinka, a humanized version of the character. If you're already doing a human character, do a furryfied version instead! :XD:
Gender Bender - Draw the character as the opposite gender.
Anti - The character's Anti-Form (Archie comics) from Moebius (or the 'normal' one from Mobius, if you're already doing a Moebian). Basically good characters become evil and the other way round, examples: Sonic ---> Evil Sonic (aka Scourge) Tails ---> Miles Eggman/Dr. Robotnik ---> Dr. Ivo Kintobor